Methane is a fungible energy source used for power production, building heating, hot water, and cooking. Methane is also growing as a transportation fuel. Methane is one of the leading carbon feedstocks for production of chemicals and materials. In comparison to coal or petroleum, methane releases significantly smaller amounts of carbon dioxide per unit of energy produced.
Microbial conversion of carbonaceous feedstocks to methane by anaerobic bacteria is well known and used in many operations. Biogenic methane production is used in municipal waste treatment to convert sewage and activated sludge to methane to recover some of the energy and reduce the mass of waste sludge that has to be disposed. Methanogenesis is also used to treat waste from food, agricultural and chemical process industries to recover carbon and energy and reduce waste discharge loads and costs. In animal feedlots that are being increasingly used for poultry, swine and beef production, the wastes are digested to reduce discharge loads, recover some energy and reduce treatment costs. Numerous small scale digesters are used to treat human and other animal waste for the same reasons, especially in the rural areas of the less developed countries. In municipal solid waste landfills, biological methane production occurs after a period of time and now the more recent landfills are being designed and engineered to enhance biological methane production and recover energy values.
In all of these “conventional” methanogenesis applications the main carbonaceous feedstocks that are utilized are typically a mix of: 1) biopolymers, e.g., cellulose, hemicellulose, lignin, pectins, and the like; 2) fats and oils; 3) proteins; and 4) other soluble and semi-soluble organics. Based on numerous studies it is generally accepted that a consortia of anaerobic microorganisms e.g., hydrolytic acidogens, syntrophic acetogens, and methanogens, work together via highly self-regulated mechanisms to bring about this bioconversion.
Until recently, it was generally believed that fossilized carbonaceous materials such as coal, peat, petroleum, oil sands etc. could not be biomethanated as they have already undergone such bioconversion processes over a long period.
For centuries, coal and peat beds have been known to produce methane leading to numerous explosions and mining disasters. Recently however, technology development and commercialization work has begun to investigate whether methanogenesis can be enhanced and exploited to produce methane in more significant quantities, so that it can be used as a cleaner energy resource than coal with lower emissions of carbon dioxide and of other environmental pollutants such as mercury and SOx during combustion. Cultures of bacteria capable of biomethanating coal and other carbonaceous feedstocks have been identified in produced waters, manure pits, digesters, activated sludge from waste water treatment plants and as isolated cultures.
Methane, the primary component of natural gas, is perhaps the most desirable fossil fuel. It is thermodynamically stable, has very high energy content, and is readily transportable with existing pipeline infrastructure. It is currently used in almost all energy applications, even as a transportation fuel. Methane is used to produce most of the world's ammonia as well as many other chemicals. In many parts of the world such as the U.S. natural gas production has not kept up with increased demand for this fungible energy source, which has a smaller carbon footprint than other fossil fuel sources. One growing source of natural gas is coal bed methane (CBM). Regions with CBM, such as the Powder River Basin in Wyoming and Montana, have a well established infrastructure for collecting and distributing natural gas. These areas also have large coal deposits. Regions that have tar sands and oil shales also have the infrastructure for natural gas recovery.
Methanogenic bacterial consortia naturally produce methane from coal and other carbonaceous sources. The energy content of the coal is conserved in the methane. To balance the redox equation, CO2 (as bicarbonate) is produced concomitantly, as described below:2CH (coal)+2H2O→CH4 (for energy)+CO2 (as HCO3−)  (1a)For dry, ash-free Illinois coal the formula is C1H0.847N0.017O0.081S0.017Cl0.00015  (1b)This is a natural process that occurs in coal beds where coal bed methane (CBM) is produced, generally by the action of a consortium of anaerobic bacteria typically in a biofilm around the coal surface. An abandoned coal mine will develop a methane atmosphere over about 10 years. In the West, extraction of CBM is growing rapidly, and the U.S. Department of Energy is supporting technology development to ensure that the extraction does not cause environmental damage.
A typical methane digester converting sewage or other carbonaceous feedstocks to methane produces a gas that is typically 50 to 70% methane, with the remaining 30 to 50% being predominantly CO2.
Burning of carbonaceous feedstocks releases the greenhouse gas, CO2 to the atmosphere. Many developed countries either restrict these emissions or charge a fee for the amount released. Energy sources that reduce the amount of CO2 release per unit of energy are increasingly desired.
From a CO2 emission standpoint, the moment a carbonaceous fuel undergoes combustion, either complete or partial (such as gasification), the CO2 is in a gaseous form and it can only be removed by separating it using chemical process technologies. A large amount of technical work and financial investments are being directed to improving CO2 capture and sequestration performance, safety, and reliability. These techniques increase the price of energy production.
Typically, post combustion carbon capture requires handling a very large amount of material because a mole of carbon (C) produces a mole of CO2, which represents a 3.7 fold increase in mass. The captured CO2 gas must be compressed, put in a pipeline, and transported to a site to sequester the gas such as an oil reservoir or a deep saline aquifer. Because CO2 is heavier than air, leaks in CO2 pipelines are potentially co-located hazardous to human populations. Sequestration sites might be large distances from the site where CO2 is captured and thus require a long and expensive pipeline. In addition, long term stability of sequestered CO2 is not well understood and some losses of CO2 back to the atmosphere may occur over time.
Although in situ biological methane production (e.g., in coal formations) has been investigated, the problem of carbon dioxide generation and release during biological methane production has not heretofore been addressed.